1. Field of the Invention
The present invention relates to a microactuator comprising comb electrodes for causing displacement of a movable section by means of electrostatic force. The microactuator of the present invention may be used as, for example, a micromirror device whose movable section includes a light reflecting surface.
2. Description of the Related Art
A microactuator, as one application of micromachining techniques, may be used as an optical scanning device as in a laser printer, a scanner, or a display, or as an optical switching device for switching light which goes into or out of an optical fiber, for example.
Japanese Laid-Open Patent Publication No. 2000-147419 discloses a microactuator comprising comb electrodes, which functions as an optical deflector. A movable section of this optical deflector includes a mirror surface functioning as a light reflecting surface. The movable section is supported by two beams which are provided along the same line. By utilizing an electrostatic force generated between a comb electrode which is provided on the movable section and a comb electrode which is provided on a base, the movable section is tilted, with the two beams serving as an axis of twist rotation. This optical deflector is simpler in construction than an optical deflector which rotates a polygon mirror by means of a motor, and can be formed by a semiconductor batch process. Thus, advantages such as ease of miniaturization and reduced fabrication cost are expected.
The comb electrode of the movable section of this optical deflector has protrusions functioning as comb teeth each of which extends in a direction perpendicular to the tilt axis, with grooves being formed in between protrusions. The comb electrode of the base also has protrusions functioning as comb teeth each of which extends in a direction perpendicular to the tilt axis, with grooves being formed in between protrusions. The comb teeth of the movable section and the comb teeth of the base oppose each other in an engaging fashion, with a gap being left therebetween. The movable section is reinforced by the comb teeth of the movable section protruding toward the base.
The above optical deflector is a mono-axial tilt mirror. In order to ensure that the gap between the comb teeth of the movable section and the comb teeth of the base does not change when the movable section is tilted, the comb teeth extend only in the direction perpendicular to the pivot axis.
However, the above optical deflector has the following problems.
In the above optical deflector, the comb teeth of the movable section extend only in the direction perpendicular to the tilt axis; that is, the movable section is not reinforced in any other direction. As a result, the movable section has a low rigidity. If the rigidity of the movable section is low, in the case where an Au film or a dielectric multilayer film is formed on the surface of the movable section in order to enhance the light reflectance, the movable section may be warped due to a difference between the coefficients of thermal expansion of the material of each such film and the material of the movable section, thus degrading the planarity of the light reflecting surface.
The rigidity of the movable section might be increased by increasing the height of the comb teeth. However, if the height of the comb teeth is made so high as to attain a necessary level of rigidity for the movable section, the mass of the movable section will also increase, thus requiring a greater driving force for driving the movable section. Moreover, the increased mass of the movable section also leads to a lower resonance frequency of the movable section, so that it becomes difficult to drive the movable section at a fast speed.
The movable section of the above microactuator only performs a mono-axial tilt. In another type of microactuator, comb teeth are provided so as to extend along a plurality of directions, thus enabling the movable section to be tilted in a multi-axial manner. Such a microactuator has the following problems. If the comb tooth height is increased in order to enhance the rigidity of the movable section, the comb teeth of the movable section and the comb teeth of the base may come in contact with each other when the movable section is tilted. If the gap between the comb teeth is increased in order to prevent such contact, the resultant electrostatic force will be decreased. If the comb tooth height is increased, the mass of the movable section will be increased, thus leading to the aforementioned problems of a large driving force being required and the inability to drive the movable section at a fast speed. Thus, there is a trade-off between the rigidity and the mass of the movable section, and also between the rigidity of the movable section and the electrostatic force.